The invention relates to an apparatus and method for folding of print sheets or signatures.
For the further processing of printed-on sheets of paper, so-called print sheets which comprise two or more printed pages, these print sheets are folded over at least once under the effect of pressure to form a sharp edge, meaning they are folded to the format of the finished printed product along a previously perforated or grooved folding line, or also along a non-prepared folding line, according to a predetermined folding pattern. Relative to the feeding direction for the print sheets, devices for the cross-folding as well as the longitudinal folding are used for this, wherein a single device, multiple devices or also a combination thereof can be arranged in a folding machine. The folding can be realized with the aid of a so-called pocket folding and/or sword folding principle. Regardless of the type of apparatus that is used and the respective method, the precision of each individual fold is decisive for the quality of the printed product to be produced.
German patent document DE 3544495 A1 discloses a folding apparatus for cross-folding which operates based on the sword-folding principle. With this apparatus, respectively one print sheet is supplied with the aid of feed rollers in a feeding direction to a folding table and against an end stop arranged thereon. The folding table contains an opening, embodied transverse to the feeding direction, above which a mechanically operated folding sword is arranged. Two folding rollers are arranged below the folding table, in the region of the opening and parallel thereto. As soon as the front edge of the print sheet to be folded impacts with an end stop that projects upward from the folding table, as seen in feeding direction, this triggers the lowering of the folding sword onto the print sheet, positioned on the folding table. The folding sword presses against the folding line, meaning against an imaginary straight line on the print sheet along which the sheet is to be folded, and thus moves the print sheet through the opening of the folding table and toward the folding rollers. The print sheet is then gripped by the folding rollers and pulled into the folding gap, formed between the rollers, and is then folded and also compressed along the folding line. The print sheet folded in this way is subsequently conveyed away in a downward direction with the aid of the folding rollers. Furthermore known are folding machines having a folding sword arranged below the folding table and folding rollers arranged above the folding table for which the folded print sheet is respectively conveyed away in an upward direction.
Regardless of the specific arrangement, a mechanical sword of this type requires very precise and involved geometric adjustments. Adapting the length of the folding sword to the format of the print sheets can furthermore be realized only with great expenditure. In addition, a mechanical folding sword requires a relatively large structural area, but is nevertheless hard to access. Owing to the required high folding capacity, the folding sword must move at the highest possible speed and thus impacts at a relatively high speed with the print sheet. To avoid contact with the folding rollers and to clear the space on the folding table as quickly as possible for supplying the following print sheet, the folding sword must reverse its movement direction just prior to reaching the folding rollers, so as to move in the direction opposite to the previous lowering movement. The print sheet to be folded is therefore in an undefined movement shortly before takeover through the folding rollers. The print sheet consequently can deviate from the specified movement path and may not be gripped by the folding rollers, thereby resulting in the continuous danger of jamming of the successively supplied print sheets. In addition, print sheets arriving in a position that is offset from the folding position can be folded only with high technical expenditure along the predetermined folding line. Finally, the forming of so-called dog ears, meaning the folding over of the ends of the print sheets at high speeds, can hardly be controlled because corresponding guide elements are only conditionally usable owing to the space required for the mechanical sword. Accordingly, the danger of reducing the quality of the later printed product can be reduced with high technical expenditure, but cannot be eliminated.
When using an also known rotating folding sword, which is suitable for high folding capacities, the folding point in time can hardly be varied because of the mass inertia of the apparatus.
German patent document DE 10238502 A1 discloses a suitable method and apparatus for the cross-folding or the longitudinal folding of respectively one print sheet, wherein a pneumatic sword is used instead of a mechanical one, meaning a compressed air device consisting of a tube with preferably downward directed exit openings. The print sheet is supplied to this apparatus while positioned in a guide plane and is thus made available in a folding position where the predetermined folding line is located below the exit openings of the compressed air device. At that moment, a control unit transmits a trigger pulse to the compressed air device for issuing a strong blast of compressed air, e.g. measuring several hundred Bar, which is directed toward the folding line on the print sheet. As a result of the effect of this compressed-air blast, the print sheet is guided along its folding line into the folding gap between the folding rollers. Since a non-defined movement path of the print sheet is precluded in this way, the danger of a lower quality of the final printed product can thus be reduced as compared to using a mechanical sword.
Owing to the design and arrangement of the compressed air device, however, this solution does not permit an adaptation to changed formats of print sheets to be folded successively. Thus, a print sheet that follows a small format print sheet could already be in the region of the compressed air device before the previous print sheet has been conveyed further to the folding rollers. The compressed air device would thus have to be exchanged which, however, would be time-consuming and costly. Alternatively, the spacing between successively following print sheets could also be adapted to the changing format. However, this would require higher transporting speeds and thus also result in disadvantages for the processing.
When creating a cross fold immediately prior to creating a longitudinal fold, the cross folding operation results in a gap between the folded print sheets, with the mechanical as well as with the pneumatic sword, wherein this gap corresponds to the length of the respective print sheet. When using a variable format, meaning sheets having a variable length which are to be folded successively, the relatively stable folding edge of the cross fold cannot be moved to a fixed position that is defined as advantageous for transferring the print sheet to the following processing machine because the folding edge of a following, shorter print sheet is otherwise already located in the sword folding region, thus making it impossible to fold only the preceding print sheet. As a result, an involved and adjustable end stop is necessary. In addition, the lowered mechanical sword as well as the compressed air flow from the pneumatic sword, which is directed onto the print sheet to be folded, prevents an immediate feeding of the following print sheet and thus a quicker production.
Especially for the further processing of sheets that are printed sequentially with the aid of digital printers, which can be used to print the sheets in the predetermined sequence for the finished printed product and thus allow producing the products in relatively small piece numbers up to a single copy, successively following print sheets frequently have different formats which can be folded with the apparatuses and methods known from the prior art only after the compressed air device has been exchanged or the spacing between successively following print sheets has been adapted. In addition to the aforementioned disadvantages of the pneumatic sword, this also makes more difficult or impossible an automation of the folding process.
Finally, digital printers for transferring the print image directly from a computer to the printer and without the use of static print forms are nowadays used to print increasingly higher numbers of print material per time unit. As compared to the past, this poses clearly higher requirements for the quality as well as the capacity of the devices used for the processing, for example the folding devices.